Generally, the present invention relates to projection screen structures for use in systems requiring spatial alignment of multiple images by using calibration points defined on a screen.
For large light displays, projection display systems typically employ more than one projector (television or film) to provide wide field-of-view imagery with the required combination of resolution and brightness. In various systems, the projector either may move or remain stationary. In either event, the projectors, and their imagery, should have good spatial alignment to provide seamless transition of imagery between projectors.
To accomplish spatial alignment between projectors, it has been proposed to provide marked calibration points on the screen. Generally, the markers for calibration points should meet certain requirements. Specifically, the markers should provide sufficient contrast with the projected imagery so as to be clearly and distinctly seen. Also, the markers should be small enough not to be obvious in the projected scene (create a distraction). Additionally, it should be possible to "turn off" the contrast of the calibration markers when the calibration is completed.
Historically, calibration points have been marked by small lights, e.g., Light Emitting Diodes (LED's) or the terminal ends of fiber optics bundles illuminated from an external source. Although such devices are satisfactory for alignment operations, their installation and use involve considerable hardware and sizable labor costs. Specifically, to install the light devices, initially holes must be drilled through the display screen structure, e.g. dome, so that an illuminator device (LED or fiber optic) can be fixed in position. In view of the size of the screen structures, scaffolding usually is required for access to calibration points to install the illuminator devices. As a further inconvenience, typically, one person is required to drill the holes while a second person (using a surveying instrument) is required to guide the first person in locating the precise position of the holes. Each illuminator device then must be mounted and interfaced to a power or light source, often requiring complex wiring or fiber optics, electrical interfaces and controls. Accordingly, a substantial need exists for an improved process and structure for accomplishing calibration-point markers on a projection surface.
Generally, the system of the present invention involves the utilization of directional reflecting materials (retro-reflective materials) to mark calibration points for image alignment. As a characteristic, retro-reflective materials return most of the illumination directly back to the originating light source. Forms of retro-reflective materials are well known and are used in industry as for signs, safety reflectors and so on. Note that the characteristic of the material is quite independent of surface tilt relative to the incident light over large incidence angles. If an observer is close to the source, the returned illumination is quite high. However, if the observer displaced from the source, the illumination is quite dim.
In accordance herewith, as disclosed in a detailed embodiment, small dots of retro-reflective material are adhesively secured to mark calibration points on an image screen. As indicated, during normal viewing, it is desirable that the contrast between such dots and the projected scene not detract from the scene content. In that regard, the physical arrangement of many projection systems is such that the viewer is well removed from the projector. That is, for many areas of projection displays, there is sufficient separation between the projector and the viewer (demanded by the physical size of the projection hardware) that the viewer will not perceive the retro-reflective dots. Accordingly, the reflected projector illumination from the retro-reflective dots is quite dim and somewhat invisible to the viewer in the projected scene. Stated another way, if the gain (from the viewer location) of the dots is less than that of the surrounding screen material, the dot brightness will be lower than the surrounding scene imagery and not be noticeable.
During spatial alignment or calibration, the viewer can wear a headlamp providing a light source near the viewer's eyes. Consequently, the separation or angle between a vector from the light source to the calibration mark and a vector from the reflectance to the viewer's eyes would be small. Therefore, the reflectance from the markers would be high and easily visible to the viewer in the projected scene.
In some projection systems, as explained in detail below, retro-reflective markers at some locations may present a distraction. Such calibration points may be identified as disclosed herein and an alternative form of device, e.g. an LED illuminator, may be employed.
To place retro-reflective markers on the screen, without the necessity of scaffolding, a placement tool in accordance herewith is disclosed below. Generally, individual markers, for example in the form of small dots, are punched from a sheet of medium, then positioned to be adhesively secured at an identified location. Accordingly, depending on the nature of the projection system, the size of the screen, the number of projectors and other criteria, the system hereof affords an economical and convenient technique and apparatus for marking calibration points to spatially align images.